Dr. Robert Huizenga, an associate clinical professor of medicine at UCLA whom The Biggest Loser fans will recognize from appearances on the show, exclusively provided ET with an update on the actor's health after Sheen revealed his HIV-positive status last November.

"I understand he's doing phenomenal," he said. "He's on a new experimental medication for HIV."

When the former Two and a Half Men star disclosed his HIV status last year, he said, "I think I released myself from this prison today." At the time, Huizenga also noted that the 50-year-old actor's viral load was undetectable.

Now, Huizenga praises Sheen's advocacy, telling ET, "He is living up to his promise to use his celebrity status not only to alert the country and the world of these incredible needs for better HIV treatment -- especially in the over two-thirds or three-quarters that aren't getting proper HIV medication -- but he's also kind of pioneering the way with new treatments."

"I'm really proud of everything he's done and his continued commitment to HIV education," he said.

Sheen has publicly struggled with substance abuse issues in the past and spent several stints in rehab facilities, but wrote in an open letter, "My partying days are behind me. My philanthropic days are ahead of me."